In general, silver halide grains are prepared by reacting an aqueous silver salt solution and an aqueous halide solution in an aqueous colloidal solution in a reactor. Precisely, there are known a single-jet method where a protective colloid such as gelatin and an aqueous halide solution are placed into a reactor and an aqueous silver salt solution is added thereto with stirring for a certain period of time, and a double-jet method where an aqueous gelatin solution is placed into a reactor and an aqueous halide solution and an aqueous silver salt solution are added thereto simultaneously. The double-jet method offers the advantage of allowing the formation of silver halide grains with a narrow grain size distribution, and the halide composition of the grains may freely be varied at various stages of growth of the grains by the double-jet method.
It is known that the growing speed of silver halide grains varies largely, depending upon the silver or halogen ion concentration in the reaction solution, the concentration of silver halide solvent therein, the distance between grains and the grain size. In particular, non-uniformity of the silver ion or halogen ion concentration derived from the aqueous silver salt solution and the aqueous halide solution added to a reactor cause a different growing speed in accordance with the different concentration of the respective ions and which results in a non-uniform silver halide emulsion. In order to overcome such non-uniformity in the final emulsion, it is necessary to rapidly and uniformly blend the aqueous silver salt solution and the aqueous halide solution, which are added to the aqueous colloidal solution, and react them together, so that the silver ion or halogen ion concentration in the reactor is uniform. In the conventional method of adding the aqueous silver halide solution and the aqueous silver salt solution to the surface of the aqueous colloidal solution in a reactor, the halogen ion and silver ion concentration are relatively high at and near the position to which the reaction solutions have been added, so that it is difficult to prepare uniform silver halide grains by this method. In order to overcome such local elevation of the concentration, the techniques illustrated in U.S. Pat. No. 3,415,650, British Patent 1,323,464 and U.S. Pat. No. 3,692,283 were developed. In accordance with these known means, a hollow rotary mixer is provided which has slits in the cylindrical wall and wherein the inside of the mixer is filled with an aqueous colloidal. More preferably the mixer is divided into an upper and lower room by a disc. The mixer is provided in a reactor vessel filled with an aqueous colloidal solution so that the rotary shaft of the mixer is vertical to the reactor vessel. An aqueous halide solution and an aqueous silver salt solution are fed into the mixer from the top and bottom open mouths through feeding ducts while the mixer is rapidly rotated so that the solutions are rapidly blended and reacted together. When the mixer has the separating disc, the aqueous halide solution and the aqueous silver salt solution as fed into the two rooms are diluted with the aqueous colloidal solution filled in each room, and these are rapidly blended and reacted near the outlet slits of the reactor. The silver halide grains formed by the reaction are expelled out into the aqueous colloidal solution in the reactor vessel because of the centrifugal force formed by the rotation of the mixer and the grains are grown in the colloidal solution in the reactor vessel.
On the other hand, JP-B-55-10545 (the term "J-PB" as used herein means an "examined Japanese patent publication") discloses a technique of improving the local distribution of the ion concentration to prevent the non-uniform growth of grains. In accordance with the method, a mixer filled with an aqueous colloidal silver is provided inside a reactor vessel which is filled with an aqueous colloidal solution. An aqueous halide solution and an aqueous silver salt solution are separately fed into the mixer through feeding ducts so that the reaction solutions are rapidly and vigorously stirred and blended by the lower stirring blades (turbine blades) as equipped in the mixer to form and grow silver halide grains. The grown silver halide grains are immediately expelled out from the mixer by the upper stirring blades, provided above the lower stirring blades, to the aqueous colloidal solution in the reactor vessel through the opening mouth as provided in the upper portion of the mixer.
JP-A-57-92523 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") also discloses a means of overcoming the non-uniformity of the ion concentration. Precisely, a method of forming silver halide grains is described in which a mixer filled with an aqueous colloidal solution is provided in the inside of a reactor vessel filled with an aqueous colloidal solution. An aqueous halide solution and an aqueous silver salt solution are separately fed into the mixer from the opened bottom thereof, both reaction solutions are diluted with the aqueous colloidal solution and are rapidly stirred and blended by lower stirring blades provided in the mixer to form and grow silver halide grains in the mixer. The thus formed and grown silver halide grains are immediately expelled out from the upper opening mouth of the mixer to the aqueous colloidal solution in the reactor vessel. An apparatus for the method is also disclosed. The method and apparatus are characterized in that both reaction solutions diluted with the aqueous colloidal solution are passed through gaps between the inside wall of the mixer and the outer tops of the blades of the stirrer without being passed through the gaps between the blades of the stirrer so that the both reaction solutions are rapidly and vigorously sheared, blended and reacted in the gaps to give uniform silver halide grains.
In accordance with the above-mentioned methods and apparatuses, although the non-uniformity of the local concentration of silver ion and halogen ion in the reactor vessel can completely be overcome, nonuniformity of the concentration in the mixer still exists. In particular, there is a significant concentration distribution near the nozzle through which an aqueous silver salt solution and an aqueous halide solution are fed into the mixer, in the lower part of the stirring blades and in the stirring portion in the mixer. The silver halide grains fed into the mixer together with a protective colloid pass through the portion which have such non-uniform concentration distribution. Most importantly, the fed silver halide grains rapidly grow to large sizes in the portion. In accordance with the methods and apparatus, since the ion concentration distribution still is in the inside of the mixer and the grains rapidly grow in the mixer, the object of uniformly growing silver halide grains under the condition of substantially no concentration distribution can not be attained.
Further, in order to overcome non-uniform distribution of silver ion concentration and halogen ion concentration by more complete blending of the reaction solutions, a means of independently providing a reactor vessel and a mixer vessel and feeding an aqueous silver salt solution and an aqueous halide solution into the mixer vessel and rapidly blending them therein so as to form and grow silver halide grains has been proposed. For instance, JP-A-53-37414 and JP-B-48-21045 disclose a method of forming silver halide grains in which an aqueous protective colloid solution containing silver halide grains in a reactor vessel is circulated from the bottom of the vessel by a pump and a mixer vessel is provided in the course of the circulating system. An aqueous silver salt solution and an aqueous halide solution are fed into the mixer vessel, and both aqueous solutions are rapidly blended in the mixer vessel to grow the silver halide grains. An apparatus for the method is also disclosed. U.S. Pat. No 3,897,953 discloses a method of forming silver halide grains in which an aqueous protective colloid solution containing silver halide grains in a reactor vessel is circulated from the bottom of the vessel by a pump, and an aqueous halide solution and an aqueous silver salt solution are injected into the course of the circulating system by a pump. JP-A-53-47397 discloses a method of forming silver halide grains in which an aqueous protective colloid solution containing a silver halide emulsion in a reactor vessel is circulated therein by means of a pump. An aqueous alkali metal halide solution is first injected into the circulating system and allowed to diffuse therein until the system becomes uniform, and an aqueous silver salt solution is thereafter injected into the system to form silver halide grains. An apparatus for the method is also disclosed. In accordance with the methods, even if the flow rate of the aqueous solutions to be introduced into the circulating system in the reactor vessel and the stirring efficiency of the mixer vessel were controlled independently so that growth of silver halide grains could be conducted under the condition of a more uniform concentration distribution of the reaction solutions, the silver halide crystals transferred from the reactor vessel together with the aqueous protective colloid solution would rapidly grow in the portion of the inlet into which the aqueous silver salt solution and the aqueous halide solution are introduced. Accordingly, for of the same reason mentioned above, it would be impossible in practice to eliminate the concentration distribution in the mixing portion or near the inlet. That is, the object of uniformly growing silver halide under the condition of uniform concentration distribution could not be attained by the methods.